1. Field of the Invention
The invention relates in general to the fabrication of semiconductor integrated circuits (ICs), and more particularly to the fabrication of a word line in dynamic random access memory circuits.
2. Description of the Related Art
To maximize the surface area of a capacitor for a dynamic random access memory (DRAM), the capacitor is typically formed over word lines. However, because the word lines and the storage cells are both conductive layers, an insulating layer between them is necessary. Silicon oxide and silicon nitride are both widely used as insulating layers. However, using silicon nitride causes a potential stress effect on the associated transistor gate and causes difficulties in patterning the gate as well. Therefore, silicon oxide is the most often used material for this application. Deposition is frequently employed to form a silicon oxide (cap oxide) layer, in particular, low pressure chemical vapor deposition (LPCVD) at 700.degree. C. is preferred because of the resulting efficiency.
The following describes a conventional process for forming a word line used in DRAM fabrication.
Referring to FIG. 1A, a substrate 10 with gate oxide layer 14 and word line 16 is provided. A refractory tungsten silicide (WSi.sub.2) layer 18 is then formed on the word line 16, which is a polysilicon material. This approach reduces interconnect resistance. The silicides of interest can be formed by three basic techniques, each of which involves a deposition followed by a thermal step to form the silicide 18:
1) deposition of pure tungsten on the polysilicon layer 16; PA1 2) simultaneous evaporation of silicon and tungsten from two sources (co-evaporation); and PA1 3) sputter-depositing tungsten silicide, either from a composite target, or by co-sputtering or layering.
Referring now to FIG. 1B, an insulating layer 20 must be formed on the surface of the tungsten silicide layer 18 to isolate the tungsten suicide layer 18 from subsequently deposited layers. Vapor deposited silicon oxide and silicon nitride can be used for this application, but a silicon oxide layer 20 formed by LPCVD at 700.degree. C. generally provides better material properties and is therefore preferred. The oxide layer 20 formed must be stable and exhibit adequate electrical and physical properties. However, in practice, the surface of the tungsten silicide layer 18 easily converts to tungsten oxide in the presence of oxygen and at a high temperature, such as, 700.degree. C. Although the tungsten oxide is initially volatile, it unavoidably remains on the top surface of the tungsten silicide layer 18 after the cap oxide layer 20 is deposited due to the efficiency of the deposition process. Therefore, the undesirable tungsten oxide (WO.sub.3) layer 22 is unavoidably formed under the silicon oxide layer 20. In other words, the tungsten oxide layer 22 is formed between tungsten silicide layer 18 and the silicon oxide layer 20. Moreover, the tungsten oxide layer 22 is not smooth, having a number of concave and convex areas, which results in the surface of the silicon oxide layer 20 being rough as well. Another effect that can be observed is that the polysilicon layer 16 becomes thicker while, by contrast, the tungsten silicide layer 18 becomes thinner. The above mentioned undesirable effects make patterning of the word line 16 rather difficult.